A plasma arc torch is defined by a cylindrical torch body and a head extending from the body. The head is constituted by an electrode positioned carefully in a cone-ended nozzle behind a nozzle orifice and a nozzle throat. The cone end may be straight walled or curvaceous. The electrode and a work piece, towards which the nozzle throat is directed, are maintained at opposite electrical polarities. Ionizable pressurized gas, typically one or more, selected from oxygen, nitrogen, hydrogen, air and argon, is constricted between the electrode and the nozzle orifice.
A power source initiates a spark between the electrode and the nozzle when the nozzle is temporarily brought in opposite polarity with the electrode. The head is then positioned towards a work piece. High-pressure gas is led into a zone between the operative front end face of the electrode, bearing an emissive insert, and the nozzle orifice. This is the plasma formation zone or the plenum. The spark ionizes a portion of the gas in this zone to, at first, enable a pilot low current arc to be formed between the emissive insert and the nozzle. The nozzle is then disconnected from the power circuit and the work piece is brought into circuit and a sustained high velocity high current plasma arc column is projected through the nozzle orifice and focussed by the nozzle throat on a selected location on the work piece. The arc melts and cuts the work piece. The accurate formation of the plasma cutting arc is dependant among other factors upon proper attachment of the arc to the center of the electrode and the careful positioning of the electrode face spaced apart from the nozzle orifice.
An accurate arc attachment point on the electrode is achieved by ensuring that the plasma arc is perfectly centered for high performance cutting. This means that the plasma beam or arc column should attach to the center of the electrode front face at the emissive insert and pass through the center of the nozzle orifice and axially through the nozzle throat. This will ensure that the cut edge has as minimum a taper as possible, there is optimum cut accuracy at optimum cut speeds and the life of the consumables like the electrode and the nozzle is maintained as long as possible.
Conventional torches use diametric location as the centering method. In conventional torches, the electrode's outer diameter is located in the swirl's inner diameter and the swirl's outer diameter is located in the nozzle's inner diameter. Since these three parts have to fit into each other and there is a clearance required between them, it is inevitable that there will be a certain amount of misalignment between the electrode face and the nozzle orifice disturbing the centering to the extent of the play.
The accurate arc attachment point on the electrode is also achieved by maintaining a strong vortex of gas around the electrode. A swirl having a plurality of passages drilled there through is provided and directs gas into the annular space between the electrode and the nozzle, which spins around the electrode vortex like and eventually arrives in the plasma formation zone or plenum between the front end face of the electrode having an emissive insert and the nozzle orifice. The vortex creates an axial suction force, which forces the arc to be centered axially through the vortex train. The vortex train further focuses the arc axially through the nozzle throat. The vortex train of gas is however confronted along its path before entering the plenum with the taper of the conical end of the nozzle. This tapered region causes the gas vortex to change direction resulting in disturbance in the alignment of the vortex axis and therefore turbulence. This turbulence is directly proportional to the speed of gas flow and its pressure. This turbulence affects the centering of the arc, which in turn affects the cutting quality and cutting speed of the plasma torch.